This invention relates to a plasma CVD device used for forming a film on a workpiece.
Recently, much attention is paid to plasma CVD, i.e., chemical vapor deposition utilizing plasma as a method for forming a film at a comparatively low temperature, for example, at a room temperature. In a typical device for carrying out such plasma CVD, as disclosed in FIG. 3 of Japanese Laid-Open Utility Model Application No. 13119/89, a first and a second planar electrodes are arranged in parallel relation within a vacuum vessel which is appropriately grounded. A high frequency power source is connected to the first electrode and the second electrode is grounded. The internal space of the vacuum vessel is provided as a reaction space. A part of reaction gas supplied into this reaction space is caused to be plasma by high frequency field generated between the first electrode and the second electrode. Negative ion or electron of all the plasma is violently motioned by this high frequency field. As a consequence, the reaction gas is partly decomposed to generate a radical. In the second electrode, a planar workpiece is mounted along the second electrode. The positive ion and radical are adhered to the surface of this workpiece, thereby to form a film.
In the above device, since the positive ion is not biased toward the workpiece, the film formed on the workpiece is low in adhesive strength and thin. Further, because of the reasons that a film is also formed on the first and second electrodes and plasma is also generated between the first electrode and the wall of a chamber faced with the first electrode, electric power is wastefully consumed.
A plasma CVD device, which is more resemble the present invention, is disclosed in Japanese Laid-Open Patent Application No. 311448/93. In this device, the internal space of the vacuum vessel is divided into plural sections by a wire net. The divided spaces are surrounded by the wall of the vacuum vessel and the wire net and provided as reaction spaces. The vacuum vessel and the wire net are appropriately grounded so that they serve as a second electrode. A first planar electrode is received in each of the reaction spaces. This first electrode is connected to a high frequency power source through an impedance matching circuit. A workpiece of a generally three-dimensional configuration is loaded on an upper surface of the first electrode. The positive ion of all the plasma generated by the high frequency field proceeds toward the workpiece due to bias function of the impedance matching circuit and the radical, accompanied by the positive ion, also proceeds to the same direction. As a consequence, a film is formed on the surface of the first electrode.
In the device disclosed by the above Japanese Laid-Open Patent Application No. 311448/93, the upper surface among all surfaces of the workpiece is faced with the second electrode. Since the positive ion and its accompanying radical collide with the upper surface generally at right angles, the film formed there is large in adhesive strength and thick. However, the film, which is formed on other surfaces of the workpiece perpendicular to the upper surface of the first electrode, especially the film formed on a lower end portion near the first electrode is small in adhesive strength and thin. The reasons are guessed that since the positive ion and its accompanying radical are drawn toward the upper surface of the first electrode, the amount of ion, etc. colliding the lower end portion of the workpiece is small and the ion, etc. are collided with the lower end portion of the workpiece diagonally instead of at right angles.
Further, a film is formed on the wide area of the upper surface of the first electrode, and the plasma generated between the lower surface of the first electrode and the second electrode faced with the lower surface does not contribute to the formation of film on the workpiece at all. Thus, electric power is wastefully consumed.